A crossbar switch is a switch which can be used to interconnect any one of a plurality of inputs to any one of a plurality of outputs. Crossbar switches can be electromechanical, electrical or optical. In principle, optical interconnect technologies offer several advantages over electromechanical and electrical systems. Thus, connections can be made at higher speeds with less crosstalk and less power consumption than electrical channels. Moreover, the power required is almost independent of the length of the connection, at least over the length of connections involved within a parallel configuration.
FIG. 1 shows a simple crossbar switch having three inputs I1, I2 and I3, three outputs 01, 02 and 03, and nine switches located at the cross points of the inputs and outputs. Clearly, by suitably controlling the switches, any input can be connected to any output.
The simple crossbar switch shown in FIG. 1 is topologically equivalent to each of the optical crossbar switches illustrated schematically in FIGS. 2 to 4. Thus, each of FIGS. 2 to 4 shows an optical crossbar switch having a localised fan-out of each input I1, I2 and I3, followed by an optical transposition, followed by a localised fan-in into each output 01, 02 and 03. In FIG. 2, the cross points (switches) are located at the inputs downstream of the fan-out. In FIG. 3, the cross points are located at the outputs upstream of the fan-in; and, in FIG. 4, the cross points are positioned in the paths of the optical transpose.
An optical crossbar switch may be a broadcast-and-select switch, that is say a switch in which signals are sent down all paths from the inputs, and selection is made at the outputs by switching devices, or a route-and-select switch, in which initial path selection is made at the inputs, and selection is made at the outputs to deflect signals to the appropriate light receptor, a respective light receptor being associated with each of the outputs.
The specification of our International Patent Application number PCT/GB01/03643 describes an optical transpose system, that is to say an apparatus for the optical transpose (or optical rearrangement) of signals. That optical transpose system has three stages, the first of which consists of an array of mesolenses that image the light from an array of light sources in an input plane, and the third of which consists of an array of mesolenses that image light onto an array of receiving devices in an output plane. The second optical stage is a macrolens placed between the two arrays of mesolenses, so as to re-arrange the beams input thereto from the first optical stage for direction to the third optical stage. The system is such that each light source is connected to a respective receiving device and vice versa, and the interconnection pattern corresponds to a transposition.